1. Field of the Invention
The present invention relates to a chain hoist motor drive controller which is housed entirely within the casing of a chain hoist to track and control the movement of the load carried by the chain hoist relative to a fixed location.
2. Description of the Prior Art
Chain hoists are utilized in many different applications to raise and lower loads suspending from overhead supports. A chain hoist is comprised of a heavy-duty motor housed within a rugged casing and having at least one chain access opening in the casing. A chain may be suspended from an overhead support or from the chain hoist itself to carry a load. In either case the chain is routed around a chain drive gear located within the chain hoist casing. The chain drive gear within the casing is driven by the chain hoist motor. The slack portion of the chain, after passing around the drive gear within the casing, is routed back out through the chain opening and hangs from the chain hoist casing as a slack end having a length that varies with the position of the chain hoist casing relative to the overhead support or with the position of the load relative to the chain hoist casing.
The chain hoist motor, through an internal chain drive gear within the casing, pulls either the load or the motor casing vertically upward, or allows the load or chain motor casing to travel vertically downward. The travel of the chain hoist casing or the load vertically up and down is controlled by switches located remotely from the chain hoist casing and coupled to the chain hoist motor by means of an electrical control cable. One or more hooks that are attached to the chain motor casing suspend a load beneath the chain hoist casing. This load is raised and lowered, under the control of the chain hoist operator, by the upward and downward travel of the load or the chain hoist along the portion of the chain which is under tension and from which the chain hoist is suspended. This travel occurs by pulling chain in and playing chain out from the casing. One such conventional chain hoist is described in U.S. Pat. No. 2,991,976, while another is described in U.S. Pat. No. 3,960,362.
Chain hoists are utilized extensively and in widely differing applications. They are used in shops, factories, warehouses, shipyards, and numerous other types of commercial and industrial establishments. In many applications of commercially available chain hoists the position of the chain hoist motor and casing relative to the length of the suspended chain upon which it travels or the position of the chain which travels relative to it may be controlled merely by observing either the chain hoist itself, or the load suspended from it. Adjustments to the vertical position of the chain or chain hoist may be performed merely by providing manual inputs to the chain hoist control switches. Indeed, a simply manually operated control is sufficiently accurate for many, many chain hoist applications that do not require precise position control.
On the other hand, there are some applications in which precision control of the chain hoist is required. In the theatrical industry, stage sets and props are often moved vertically utilizing general purpose chain hoists, but this movement must be controlled with great precision. For example, different portions of a stage prop may be moved vertically relative to the stage and relative to each other in a closely controlled and intricate sequence and at precise speeds in order to produce special theatrical effects. Precision control of general purpose chain hoists is often necessary in other applications as well. For example, precision control of a general purpose chain hoist may likewise be required at trade shows and expositions in order to create special effects or in order to move interdependent loads in a complex manner. Where precision control of a chain hoist is necessary, visual observation and corresponding adjustment utilizing manual controls is very inadequate and unacceptable.
To provide the necessary precision control for specific applications of general purpose chain hoists, various position-encoding systems have been devised. However, all of these conventional systems have certain drawbacks.
One prior art conventional system is described in U.S. Pat. No. 4,905,848. This system employs magnetic sensors on a pulley located externally of a motor in order to provide position information so as to guide the motor to move a load to a proper destination. However, errors are introduced into this system due to stretching of the cable or twisting of the chain, depending upon the load and its orientation. Furthermore, because the system operates at a single speed, there is a jarring or bounce that occurs when the load arrives at its destination and the motor is shut off.
Another prior system is described in U.S. Pat. No. 5,790,407. This system employs magnetic or optical sensors located on a cable take-up reel and entails the same problems of discrepancies between the position as sensed by rotation of the take-up reel and the actual position which is determined by rotation of the chain hoist motor shaft. This prior system also operates the motor at a single speed, either full on or completely off. Consequently, the load is subjected to a jarring or bouncing effect both when it is initially moved from a stationary position and also when it arrives at its destination.
Furthermore, the prior systems for operating chain hoists require a considerable amount of hardware and electrical components that must be packed separately and transported separately from the chain hoist itself.